The Sun

The Sun

Photosphere - the part we see
Sun’s composition is typical
 ~94% H
 ~5.9% He
Radiation from the photosphere  peaks in the middle of
the visible spectrum
 We’ve evolved over time to have our eyesight most
sensitive there
About 10,000 km (~1.5% of solar radius) thick,
directly above photosphere, is the
 jagged, spiky layer called the chromosphere (seen
pinkish during an eclipse)
Above this layer is the corona, extending tens of
millions of kms into space - solar wind
 We’re bathed in this solar wind

Photosphere

Surface temp about 5800 K
We can resolve details to about 700 km across
Granulation at surface, but current telescope
resolution isn’t quite enough to 
 distinguish details
Photosphere oscillates up and down, which tells us
about interior (temperature, 
 density, rate of rotation of interior)

Chromosphere

Look at sun through an H-filter to see this layer
Temp is between7000 K and 15,000 K - higher than
photosphere
Composed of small spicules - jets of gas rising and
falling, looking like blades of grass
 some 7000 km tall, 700 km across, lasting 5 to 15
minutes

Corona

Very  irregular in shape - streamers away from sun
Too faint to see, except during eclipses
Temp near 2,000,000 K at lowest levels
Emits mainly x-rays
There are a few “cooler” holes - out of which solar
flows to Earth
Corona is not uniform

Sunspots

11-year cycle (first noted in 1850)
Dark center - umbra
Surrounded by penumbra
Regions of high magnetic field strength (1000’s of
times stronger than that of Earth)
Discovered by Galileo in 1610 by projection of solar
image
There was an instance between 1645 and 1715 where
there were NO sunspots
 the Maunder minimum - activity is less regular than
imght be thought, though
 the evidence is a bit sketchy; perhaps Earth’s
climate affected observing
Every 11 years, Sun’s north and south magnetic poles
reverse